BuiltStrong saidOP did you carb load before the run? Your meals prior to the run will/can cause the effects you hare having. You sound as someone else said the first signs of dehydration. The night before a run try drinking Gatorade or some form of electrolyte replenisher. Also fluids during your run even a small amount will help you, especially in the heat bro.
5K is not a long run and there is no reason to carb load before a 5K run. In fact, if he runs in the morning after eating dinner the evening before, there is no reason to eat anything before running. Also, there is no need to guess about dehydration. Dehydration always
causes the urine to be very dark and sometimes stinky; if his urine is not dark, he is not dehydrated. There should be no need to drink fluids during a 5K run; that's only three miles and could be completed easily in less than 24 minutes, which is an easy eight minute per mile pace. When I was fifty, I was able to run 10 miles in slightly less than seven minutes per mile. Even a 10K (six mile) run should not require drinking fluids during the run for someone who is adequately hydrated before starting.
If he became dizzy after running more than 10K and were not in condition for that distance, I'd say that it's caused by lack of conditioning. But he's running only 5K and, from what he says, one would expect him to be in good enough condition for it.
A visit to a doctor well qualified in athletic medicine is in order.